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Re: [Qemu-block] [PATCH v4] backup: allow target without .bdrv_get_info

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH v4] backup: allow target without .bdrv_get_info
Date: Tue, 7 Mar 2017 19:39:54 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1

07.03.2017 16:47, Kevin Wolf wrote:
Am 28.02.2017 um 20:33 hat Vladimir Sementsov-Ogievskiy geschrieben:
Currently backup to nbd target is broken, as nbd doesn't have
.bdrv_get_info realization.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>

v4: use error_report()
     add article

v3: fix compilation (I feel like an idiot)
     adjust wording (Fam)
v2: add WARNING


Since commit

commit 4c9bca7e39a6e07ad02c1dcde3478363344ec60b
Author: John Snow <address@hidden>
Date:   Thu Feb 25 15:58:30 2016 -0500

     block/backup: avoid copying less than full target clusters

backup to nbd target is broken, we have "Couldn't determine the cluster size of
the target image".

Proposed NBD protocol extension - NBD_OPT_INFO should finally solve this 
But until it is not realized, we need allow backup to nbd target due to backward

Furthermore, is it entirely ok to disallow backup if bds lacks .bdrv_get_info?
Which behavior should be default: to fail backup or to use default cluster size?

  block/backup.c | 12 +++++++++++-
  1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/block/backup.c b/block/backup.c
index ea38733849..ea160e9e82 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -24,6 +24,7 @@
  #include "qemu/cutils.h"
  #include "sysemu/block-backend.h"
  #include "qemu/bitmap.h"
+#include "qemu/error-report.h"
  #define SLICE_TIME 100000000ULL /* ns */
@@ -638,7 +639,16 @@ BlockJob *backup_job_create(const char *job_id, 
BlockDriverState *bs,
       * backup cluster size is smaller than the target cluster size. Even for
       * targets with a backing file, try to avoid COW if possible. */
      ret = bdrv_get_info(target, &bdi);
-    if (ret < 0 && !target->backing) {
+    if (ret == -ENOTSUP) {
I think this should be if (ret == -ENOTSUP && !target->backing) because
the warning explicitly says "doesn't have a backing file", and the case
with a backing file is already handled below (without a warning).

I can fix this while applying if you agree.

Ok. no problem

+        /* Cluster size is not defined */
+        error_report("WARNING: The target block device doesn't provide "
+                     "information about the block size and it doesn't have a "
+                     "backing file. The default block size of %u bytes is "
+                     "used. If the actual block size of the target exceeds "
+                     "this default, the backup may be unusable",
+                     BACKUP_CLUSTER_SIZE_DEFAULT);
+        job->cluster_size = BACKUP_CLUSTER_SIZE_DEFAULT;
+    } else if (ret < 0 && !target->backing) {
          error_setg_errno(errp, -ret,
              "Couldn't determine the cluster size of the target image, "
              "which has no backing file");

Best regards,

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